1. Field of the Invention
The present invention relates to an optical information processing apparatus for optically recording and/or reproducing information by focusing a small optical spot on an information recording medium such as an optical disk or a magnetooptical disk.
2. Related Background Art
When a rewritable information recording medium such as an optical disk or a magnetooptical disk is used as the external memory of a computer, there is required high random access speed. For improving the access speed, it is important to reduce the weight of the movable part of the optical head and to reduce the vibration of the actuator during a coarse seeking operation by a linear motor or the like. The weight reduction of the movable part of the optical head is achieved by the use of a separated optical system in which a movable part, including the objective lens, mirror etc. of the optical head, is separated from fixed parts such as the laser and photosensor system, and the seeking operation is conducted by moving the movable part alone with the actuator. Also for reducing the vibration of the actuator, there are already known a method of locking the actuator during the seeking operation and a method of employing a slidable shaft actuator in which the movable part is moved with respect to a shaft positioned at the center of gravity of the movable part, and such methods are utilized for reducing the time required for tracking.
However, the optical head or the movable part thereof may still cause vibrations in the seeking operation even if such measures are taken, and such vibrations become more conspicuous in case the access of a higher speed is intended or in case a higher coarse seeking speed is used with a movable part of a lower weight.
These phenomena will be explained in more detail with reference to FIGS. 1A, 1B, 2A and 2B. FIGS. 1A and 1B illustrate a typical optical information processing apparatus in which the entire optical head is moved, during the seeking operation, with respect to an optical disk 3 constituting the information recording medium. A light beam emitted from a semiconductor laser unit 7 is converted into a parallel beam 6 by a collimating lens (not shown), and is directed toward a polarizing beam splitter 5. The light beam is deflected by the splitter 5 toward an objective lens 4, which focuses the light beam as a light spot 1' on the optical (or magnetooptical) disk 3. The light spot 1' is subjected to focus servo and tracking servo operations, achieved by receiving the reflecting light from the disk 3 by a sensor 10 through a condenser lens 9 fixed on a base plate 12 of the optical head and converting the light into an electrical signal, and the light spot is positioned, with respect to a target track, with a precision of .+-.1 .mu.m in the focusing operation and .+-.0.1 .mu.m in the tracking operation. An actuator 8 is provided for precise access movement of the objective lens 4 in the focusing and tracking directions. The above-mentioned sensor 10 serves to provide not only the servo signals but also an RF signal for information reproduction, corresponding to the reflected light. In the coarse seeking operation, the above-mentioned base plate 12 is moved, along rails 11, in the radial (seeking) direction of the disk 3. A yoke and a coil constituting a linear motor, for moving the base plate 12, namely the entire optical head, are not illustrated.
An optical information processing apparatus shown in FIGS. 2A and 2B employs a separated optical system in which the polarizing beam splitter 5, objective lens 4 and actuator 8 are mounted on the base plate 12, while other components of the optical head such as the semiconductor laser unit 7 and the sensor 10 are mounted on a fixed base plate 14 whereby a fixed part and a movable part are separated. A beam splitter 13 is provided on the fixed base plate 14 for separating the reflected light from the movable part toward a condenser lens 9. Other structures are the same as shown in FIGS. 1A and 1B.
A possible problem in such an apparatus lies in the vibration of the base plate 12 for example in the z-x plane, during the coarse seeking operation by the linear motor, due to a gap between the base plate 12 and the rails 11, or due to an elastic deformation of the base plate 12. Such vibration causes a displacement of the light spot 1' by an amount .delta..sub.zx in the radial direction to a spot position 1, due to the inclination of the base plate 12. The amount .delta..sub.zx easily amounts to several microns depending on the inclination of the base plate 12 and the position of center of the inclination, and may seriously affect the time required for releasing the actuator 8 from the locked state and aligning the actuator with the track. The vibration of the base plate 12 may also occur in the y-z plane or in the x-y plane, depending on the position of the center of the inclination. The vibration of the base plate 12 in the y-z plane displaces the light spot 1' by .delta..sub.yz along the direction of track. Such displacement will generate a jitter component, and will seriously affect the stability of the servo system in an optical information processing apparatus employing a sampling servo system.
Also the vibration of the base plate 12 in the x-y plane causes a displacement of the light spot in the radial direction or in the track direction depending on the position of the center of the inclination, and similarly affects the stability of the servo system as in the above-explained case.
Particularly the amount .delta..sub.zx in the separated optical system shown in FIGS. 2A and 2B is larger than the amount .delta..sub.zx in FIGS. 1A and 1B, thus more seriously affecting the time required for tracking. The vibration may also occur in the y-z plane or in the x-y plane.
As explained above, conventional structures have been associated with the undesirable influence on the stability of the servo system, due to the displacement of the light spot 1' to another position 1 on the disk 3 at the seeking operation of the optical head, due to the inclination or vibration of the optical head or its movable part (base plate 12).